1. Field of the Invention
This invention relates to a circuit for controlling load current from multiple power supplies connected to drive a single load.
2. Description of the Related Art
FIG. 1 shows a block diagram of a power supply circuit wherein multiple power supply units 100.sub.1 -100.sub.N drive a common load 102. The power supply units 100.sub.1 -100.sub.N may each include a power metal oxide semiconductor field effect transistor (MOSFET) having a source to drain connecting V.sub.DD to the load 102. As an alternative to MOSFETS, the power supply units 100.sub.1 -100.sub.N may also include some other suitable power or switching elements. Current provided to the load 102 in each of the power supply units 100.sub.1 -100.sub.N is controlled using a voltage bias circuit driving the gate of each of the power MOSFET transistors in the power supply units 100.sub.1 -100.sub.N. Typically, the bias circuitry is controlled so that the load current is equally shared between the MOSFETs of the power supply units 100.sub.1 -100.sub.N.
By equally distributing the load current between the power MOSFETs of the power supply units 100.sub.1 -100.sub.N, the difficulty a particular unit may have in achieving a desired current level is not taken into account. For example, suppose two power supply units share a 10 amp load. A typical current sharing scheme would force each power supply unit to provide 5 amps of load current. Now assume that a MOSFET in a first power supply unit has a drain to source resistance (Rds) that is 25% higher than the second power supply unit. The power dissipated by the first power supply unit would then be 25% higher than power dissipated by the second unit. The first unit could then run significantly hotter than the second unit, potentially causing reliability problems.